The present invention relates to a method for non-destructive stress wave testing of wood and particularly to a method of calculating the sound speed and dynamic modulus of elasticity according to the longitudinal fundamental frequency of a timber specimen, which can be applied as a measurement on production line for enhancing conventional methods.
Accordingly, destructive testing or non-destructive testing methods are usually adapted to estimate and measure the determinant factors of lumber qualities, such as moisture content, weight, wood grain, and modulus of elasticity. The foregoing non-destructive testing method is used to expertise the quality and the internal structure of lumber. For example, the wood strength is generally coinciding to the modulus of elasticity, whereby measuring the modulus of elasticity is wood strength estimated. The techniques for non-destructive testing of the modulus of elasticity include vibration, ultrasonic wave, and stress wave while these years method like Tap Tone is adapted to estimate the sound speed and dynamic modulus of elasticity by quickly measuring fundamental frequency with sound spectrum analysis. Tap Tone technique, as FIG. 3 shows, consists the follows steps. Sound receiver microphone 6, electrically connected to FFT spectrum analyzer 7, receives the sonic wave that is generated by the impact of hammer 9 on one end of timber specimen 8. FFT spectrum analyzer 7 will then measure the fundamental frequency with sound spectrum analysis, thereby calculating the sound speed and dynamic modulus of elasticity.
Only in quiet surroundings is Tap Tone technique an effective method for estimating the sound speed and dynamic modulus of elasticity of timber specimen 8; otherwise, the background noise causes microphone 6 to pick-up the feedback and causes FFT spectrum analyzer 7 to show inaccurate data. Except in a soundproof laboratory, Tap Tone method is unable to be adapted on general production line because microphone 6 may receive both the sonic wave of timber specimen 8 and the noises of the factory, thereby misleading the accuracy of the sound spectrum analysis. As a result, the conditions of background surroundings limit the usage of Top Tone technique.
It is, therefore, the main object of the present invention to calculate the sound speed and dynamic modulus of elasticity according to the longitudinal fundamental frequency of the timber specimen.
It is another object of the present invention to enable the present invention being used on production line for enhancing the conventional Tap Tone technique.
The foregoing object is accomplished by providing a method for non-destructive stress wave testing of wood, wherein a timber specimen is placed steadily on a sensor near the center with friction, acting on the interfaces of the sensor and the timber specimen, which resists said timber specimen from moving.
Pounding one end of said timber specimen with a striking instrument sends a compression pulse stress wave down the post toward the other end, thereby generating a tension pulse wave back along the post till friction exponentially dissipates the energy of the stress waves.
The changing signals of friction received by a receiver, electrically connected on said sensor, support an analyzer, electrically connected to said receiver, to measure longitudinal fundamental frequency of said timber specimen; therefore, sound speed and dynamic modulus of elasticity are then calculated.